Electrical submersible pumps (“ESP”) are used for pumping high volumes of well fluid, particularly in wells requiring artificial lift. The ESP typically has at least one electrical motor that normally is a three-phase, AC motor. The motor drives a centrifugal pump that may contain a plurality of stages, each stage comprising an impeller and a diffuser that increases the pressure of the well fluid. The motor is filled with a dielectric lubricant or oil that provides lubrication and aids in the removal of heat from the motor during operation of the ESP. A seal section is typically located between the pump and the motor for equalizing the pressure of the lubricant contained within the motor with the hydrostatic pressure of the well fluid on the exterior. The seal section is filled with oil that communicates with the oil in the motor.
The ESP is typically run within the well with a workover rig. The ESP is run on the lower end of a string of production tubing. Once in place, the ESP may be energized to begin producing well fluid that is discharged into the production string for pumping to the surface.
During operation, the temperature of the oil in the motor of the ESP increases due to mechanical friction and electrical efficiency in the motor. Internal motor temperature is dissipated thru the stator to the housing of the motor to the produced (pumped) fluid. Higher fluid velocity around the motor, or lower fluid temperature, can lead to increased heat removal from the motor. The internal oil has lubricant properties and in some way helps dissipate the heat from internals of the motor through heat transfer, but its effect is limited. One of the most important properties of the oil is to lubricate the bearings of the motor. The oil is also vital in dissipating heat from the bearings and thrust load bearings as well as in maintaining the motor within its rated temperature, and maintaining reliability. However, rejection of heat from the oil to the surrounding well fluid is usually limited due to the well fluid's high temperature, and also poor heat transfer characteristics due to high viscosity. The increased temperature of the motor oil may lead to low performance or premature failure of the motor.
A technique is desired to improve motor cooling by circulating oil or lubricant out of the motor to cool down the motor temperature. Thus allowing the motor to operate at a lower temperature that may translate to extended life and increased reliability of the motor.